It looks, at a glance, like a natural feature of the lake, which is partly the point. This is a crannog, an artificial or semi-artificial island dwelling used across Ireland and Scotland from the Bronze Age through to the early modern period, typically constructed from timber, peat, brushwood, and stone piled into shallow water to create a secure, defensible platform for habitation. What makes Toome's example unusual is what is no longer there: despite the name, no trace of any wooden structure survives, leaving behind only a circular cairn of stone, roughly nineteen metres across and two metres high, partially built onto a natural rock outcrop that breaks the surface at the southern end.

The lake itself has changed considerably since the crannog was in use. Toome Lough, now a sub-rectangular body of water measuring approximately 450 metres on its longest axis, would once have extended further to the south-east, into what is today a marshy area. At its fullest extent the lake may have stretched to around 600 metres in that direction. The crannog is captured on the 1907 edition of the Ordnance Survey six-inch map as a small island, suggesting that by the early twentieth century it was still clearly distinguishable as a discrete feature. The gradual retreat of the lake has since altered the relationship between the mound and the water around it, and what was once surrounded and isolated is now considerably more accessible from the shore than it would have been for its original occupants. That sense of deliberate separation, of choosing water as a boundary, is perhaps the most legible thing left about the site.